The walls of oil and gas formations are exposed during the process of drilling a borehole. The successful completion of a wellbore requires the deposit of a low-permeable filtercake on the walls of the wellbore to seal the permeable formation exposed by the drilling bit. A filtercake can limit drilling fluid losses from the wellbore and protect the natural formation from possible damage by the fluids permeating into the wellbore. Solids in the drilling fluid may also damage the formation, particularly drilling fines. The suspension of fine particles that enters the formation while the cake is being established is known as "mud spurt" and the liquid that enters subsequently is known as "filtrate". Both filtration rate and mud spurt must be minimized when penetrating potentially productive formations because productivity may be reduced by any one of the following: the swelling of clays in the formation when they come in contact with the filtrate; particles transported into the pores of the formation that plug flow channels and greatly reduce the permeability of the rock; and the pressure of some reservoirs that is not great enough to drive all of the aqueous filtrate out of the pores of the rock when the well is brought into production.
Contamination of the formation with drill solids in the fluids can be avoided by formulating the drilling fluid with other soluble solids that can be incorporated in the filtercake and dissolved at a later time, thereby diluting the effect of the insoluble solids. Sized salt solids, in salt saturated solutions, and finely ground calcium carbonate are examples of solids purposely added to the drilling, workover, or completion fluids to form a filtercake that can later be partially dissolved by aqueous or acid flushes.
For a filtercake to form, the drilling fluid must contain some particles of a size only slightly smaller than the pore openings of the formation. These particles are known as bridging particles and are trapped in surface pores, thereby forming a bridge over the formation pores. Filtercake building fluids can also contain polymers for suspension of solids and for reducing liquid loss through the filtercake by encapsulating the bridging particles. These can be either natural or synthetic polymers. The polymers can include one polymer such as xanthan selected for its Theological properties and a second polymer, a starch for example, selected for reduction of fluid loss.
At completion of the drilling or other well servicing, the filtercake must be removed to allow production of the formation fluids or bonding of cement to the formation at the completion stage. Removal of the deposited filtercake should be as complete as possible to recover permeability within the formation.
Previous chemical treatments for filtercake removal have employed an acid to dissolve carbonates and/or hydrolyze polysaccharide polymers. Dobson, Jr. et al., U.S. Pat. No. 5,607,905, reveal a process for enhancing the removal of filtercake by the use of inorganic peroxides as oxidizing agents. The process disclosed in the '905 patent incorporates alkaline earth metal peroxides, zinc peroxides or a mixtures thereof within the filtercake as an integral component thereof and then contacts the filtercake with an acidic solution. Weir et al., U.S. Pat. No. 1,984,668, disclose a method of cleaning the walls of mudded boreholes. A first reagent is included in the mud-coating of the borehole. Subsequently, the mud-coating is impregnated with a second reagent that reacts with the first reagent. The first reagent is generally a carbonate. The second reagent is generally an acid that reacts with the carbonate to form carbon dioxide gas.
Tjon-Joe-Pin et al., U.S. Pat. No. 5,247,995, disclose a method for removing a polysaccharide-containing filtercake formed on surfaces during fracturing with a viscosified fluid. The '995 method discloses removal of the filtercake by pumping downhole an aqueous fluid containing enzymes that degrade polysaccharide. The use of non-metallic persulfates as an oxidant in filtercake removal systems is also disclosed.
A composition and method of removing polymeric material from a formation is found in Hanlon, U.S. Pat. No. 4,609,475. The method disclosed comprises contacting the formation with an oxidizing agent and carboxylic acid. A source said to be effective in promoting the decomposition of the oxidizing agent is added to the aqueous solution containing the oxidizing agent.
The addition of an alkali metal or alkaline earth metal salt of hypochlorous acid, or a chlorinated isocyanurate is disclosed in Langemeier et al., U.S. Pat. No. 4,941,537, for the reduction of viscosity of aqueous fluids thickened with a tertiary amino polygalactomannan.
Methods of filtercake removal that incorporate a solid oxidizing agent precursor in the filtercake are not totally insoluble at the temperatures of application so that some oxidizing materials are released with premature degrading of the polymer. Also, the materials are only applicable in high pH fluids, thereby limiting their use. Finally, these compositions cannot be used in formulations with significant amounts of reducing agents, such as iron control agents, oxygen scavengers, or bromide or formate brines.
A problem associated with the use of acid to activate a precursor in field use is that sufficient acid does not always contact all parts of the filtercake. Sometimes the acid must be "weighted up" (formulated with high weight brines to increase fluid density). Thus the actual acid content is low. As the acid is reacting with the carbonates in the filtercake, a stoichiometric amount of acid is required, and this volume can be greater than the actual exposed wellbore volume; the acid must be continuously replaced. In some cases, the acid initially dissolves a small region of the filtercake, because of the vastly increased fluid loss of this region, the remaining acid flows to this small region and out into the formation. What is needed is a method that attacks the filtercake in a manner which does not completely remove the filtercake in a small region and does not require large stoichiometric amounts of agent.
In the drilling and completion of a well, several operations after the drilling process are often required before production can begin. These operations require the displacement of the drilling fluid without degradation or removal of the filtercake. What is needed, therefore, is a method for removal of filtercake that does not depend on the presence of a drilling fluid.
In the case of horizontal open hole drilling of unconsolidated formations, it is desirable to gravel pack the wellbore after drilling the zone but before the filtercake is completely removed. The problem is that the act of gravel packing the wellbore annulus further limits acid contact with the filtercake, as it both reduces the physical volume of acid in the zone and blocks its flow.
Consequently, there remains the need for a filtercake removal technology that does not depend solely on acid hydrolysis for removal of the filtercake. And there is an additional need to provide a method for oxidizing filtercake materials in which the oxidizing agent is not activated prematurely, i.e. at ambient temperatures, so that the oxidizing agent works in concert with drilling fluid formulations that have easily oxidized materials.